Certain redox active materials display different colors in different oxidation states. This phenomenon is called electrochromism, and the materials are called electrochromic. Electrochromism has a potential application to light modulation including, for example, displays, mirrors of variable reflectance, sunglasses, automotive windshields, sunroofs, building windows and the like.
As disclosed in co-owned U.S. Pat. No. 5,189,549, the disclosure of which is incorporated herein by reference, it is desirable for an electrochromic device to include two electrochromic materials, with "complementary" electrochromic and electrochemical properties: That is, the first electrochromic material should undergo a colorless to colored transition oxidatively, while the second electrochromic material should undergo the same color transition reductively. Furthermore, the materials are electrochemically complementary so that one provides for a source and a sink of electrons within the same system, so that electrolytic decomposition of the solvent or the supporting electrolyte is prevented. In this way, one realizes double the optical effect per electron transferred, since two materials change color to a more highly colored (darker) state simultaneously. This "complementary counterelectrode" technology is, accordingly, the approach of choice.
Three distinct types of electrochromic devices are recognized in the art: (a) the solution type, (b) the precipitation type, and (c) the thin film or electrode surface confinement type.
In the solution type of electrochromic devices, the electrochromic materials are dissolved in the electrolyte and they move to the electrodes by diffusion. Faradaic current through the electrodes causes electrolysis of the electrochromic materials to their colored redox forms, which diffuse back into the electrolyte. The greatest advantage of the solution type of electrochromic devices is the variety of materials that can be used; every single redox active material which is electrochromic is a potential candidate. Three serious drawbacks, however, of this approach are: first, the speed of coloration of this type of electrochromic devices is relatively slow because it is controlled by diffusion in the bulk electrolyte; second, the color intensity depends on the concentration of the electrochromic materials, which, in turn, depends upon their solubility in the electrolytic solution; and third, faradaic (i.e, electrolytic) current has to be sustained continuously because the color bearing redox forms of the two electrochromic materials can either annihilate each other when they meet in the bulk solution, or they can be oxidized or reduced back to their colorless states at the opposite electrodes from the ones where they were formed. The latter drawback might become a significant problem in large area light modulation applications such as automatic windshields, automotive sunroofs, building windows, etc., because of the high energy consumption associated with it.
In the second type of electrochromic devices, the precipitation type, one redox form of at least one of the electrochromic materials is originally dissolved in the electrolyte, but upon oxidation or reduction, the "colored" product is plated onto the electrode. Typical examples of this are the reversible plating of silver, or the reversible plating in an aqueous electrolytic solution of a salt of the monocation radical of the one electron reduction product of N,N'-diheptyl-4,4'-bipyridinium dication. The precipitation type electrochromic devices are still rather slow because they are controlled by diffusion in the bulk electrolyte at least towards one of the redox directions, but reversible plating of at least one of the electrochromic materials onto the corresponding electrode decreases power requirements, and can be the basis of high resolution displays.
Finally, the thin film or electrode surface confinement type of electrochromic device, in principle, alleviates all the problems associated with the other two types of electrochromic devices. In theory, the electrode surface confinement of both electrochromic materials would provide the highest resolution possible, and hopefully, it would change the charging (switching) speed from diffusion controlled to charge transfer controlled. Moreover, physical separation of the two electrochromic materials would prevent annihilation of the colored forms, thus providing the so-called open circuit "memory effect" that would significantly decrease the average power consumption. In a sense, a surface confined type of electrochromic device can be considered as a rechargeable battery, in which the color of the electrode depends upon the state of charge.
Prior efforts have been made in use of surface confined electrochromic materials in electrochromic devices, primarily using certain metal oxides and conducting polymers. With respect to the metal oxides, the electrochromic effect displayed by WO.sub.3 has attracted much interest. Reduction of WO.sub.3 films on electrodes forms the so called tungsten bronzes which are blue and electrically conducting: EQU WO.sub.3 +nM.sup.+ +ne.sup.- .revreaction.M.sub.n WO.sub.3 (M.sup.+ =H.sup.+, Li.sup.+, Na.sup.+ etc.)
This reduction depends on the availability and uptake of both M.sup.+ and e.sup.-. Therefore, in aqueous electrolytes and at a fixed pH, WO.sub.3 in the reduced state behaves as an electronic conductor below a certain potential threshold.
Despite the attention given metal oxides, such as WO.sub.3, as electrochromic materials, metal oxides tend to switch slowly, and generally have a limited cycling lifetime.
Conventional electrode surface confined redox conducting polymers, such as polyaniline, polypyrrole, etc., are electrochromic, switch fast, and due to their flexible structure, can accommodate easily the volume changes induced upon oxidation and reduction, thus offering a potentially extended cycling lifetime. However, many conducting polymers, such as polyaniline, polypyrrole, polythiophene, etc. do not tend to absorb strongly in their colored states at a film thickness which retains fast switching speed and strong adhesion to the electrode surface.
In co-owned U.S. patent application Ser. No. 717,892, the disclosure of which is incorporated herein by reference, it is disclosed that the electrochromic effect of redox conducting polymers can be improved by incorporating in them other electrochromic materials such as prussian blue (Fe.sub.4 [Fe(CN).sub.6 ].sub.3). According to this method, a redox conducting polymer such as polyaniline, polypyrrole and poly(3-methyl)thiophene is electrodeposited onto an electrode. Such conducting polymer layers are then used as electrodes with accessible internal surface area, which is electrochemically plated with prussian blue from its precursors K.sub.3 [Fe(CN).sub.6 ], and FeCl.sub.3, creating a composite electrochromic material. Reduction of the resulting conducting polymer-prussian blue composites is chemically reversible, and gives a colorless film due to formation of the colorless Everitt's salt (K.sub.4 Fe.sub.4 [Fe(CN).sub.6 ].sub.3). Incorporating prussian blue into conducting polymers not only increases absorbance of the composite film in the oxidized state, but surprisingly, it also increases the cycling lifetime of prussian blue.